Nitride semiconductors have features such as a high saturation electron velocity and a wide band gap, and their application to high-withstand-voltage, high-power transistors has been studied. For example, gallium nitride (GaN), which is a nitride semiconductor, has a band gap of 3.4 eV, which is greater than the band gap of Si (1.1 eV) and the band gap of gallium arsenide (GaAs) (1.4 eV), and has a high breakdown field strength. Therefore, nitride semiconductors such as GaN are extremely promising as materials for semiconductor devices for power supply that operate at high voltage and output high power. With respect to semiconductor devices using a nitride semiconductor, many reports have been made on field-effect transistors, particularly high-electron-mobility transistors (HEMTs).
Specifically, HEMTs using a nitride semiconductor have a structure where an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterointerface or an indium aluminum nitride/gallium nitride (InAlN/GaN) heterointerface is formed on a substrate. As a result, a high density two-dimensional electron gas (2DEG) is generated near the heterointerface. Therefore, this 2DEG can be used as a channel carrier. GaN, which is a wide gap semiconductor having a high saturation electron velocity and a high dielectric breakdown voltage, can form a high-power high-frequency device of high efficiency, and is used for, for example, high-power, high-frequency amplifiers. A nitride semiconductor layer is epitaxially grown by metalorganic vapor phase epitaxy (MOVPE).
According to HEMTs using a nitride semiconductor, an off-state leakage current may flow between a source electrode and a drain electrode through a buffer layer when a pinch-off occurs. Therefore, according to some nitride-semiconductor HEMTs, the buffer layer is doped with carbon (C) or iron (Fe).
Specifically, in the case of forming GaN by MOVPE, the capturing of donor impurities such as oxygen or donor-type defects such as nitrogen vacancies are likely to occur. Therefore, GaN is likely to present n-type conduction characteristics without being intentionally doped with impurities. Accordingly, even when an off voltage is applied to the gate, it is possible that a depletion layer does not sufficiently extend to allow a leakage current to flow via a buffer layer. Therefore, there is the technique of suppressing the off-state leakage current by increasing the resistance of the buffer layer by doping the buffer layer with Fe, which is an acceptor impurity, to compensate the donor. In particular, because Fe forms a deeper acceptor level than C in GaN, a higher effect can be expected.
Fe, however, is likely to diffuse upward in a GaN layer during its growth. Fe is greater in ion radius than Ga. Therefore, the Ga site, where Fe is to be substitutionally placed, is not easily entered by Fe, and it is believed that Fe that is not accommodated in a predetermined position moves upward to cause the above-described upward diffusion. If Fe diffuses to a region where a 2DEG is generated, the diffused Fe serves as a scattering source for electrons to reduce the electron mobility, thus causing, for example, an increase in the on-resistance.
To prevent such Fe diffusion, a structure where an AlGaN layer is formed on an Fe-doped buffer layer and an i-GaN layer serving as a channel layer is formed on the AlGaN layer is proposed. According to this structure, because the a lattice constant of AlGaN is smaller than the a lattice constant of GaN, it is possible to prevent entry of Fe that is greater in size than Ga and thus to prevent the upward diffusion of Fe.
Reference may be made to, for example, Japanese Laid-open Patent Publication Nos. 2013-211363, 2014-136658, and 2014-209638 for related art.